The fuel cell is the best alternative to compensate for today's energy demand, but the high cost of fabrication of membranes (e.g., Nafion) hampers the widespread commercialization. Plant-derived nanocellulose is renewable, most abundant, and biocompatible with high strength and tunable surface chemistry. Here we have demonstrated the jute derived-nitro-oxidized carboxycellulose nanofibers (NOCNFs) as a viable and sustainable substitute for synthetic ionomer membranes used in proton exchange fuel cells (PEFCs). NOCNFs were obtained in two functionalities: carboxylate and carboxylic acid which were then transformed into nanopaper I and II, respectively. This is the first report where NOCNFs with two different functionalities were tested in PEFCs. The results indicated that nanopaper II performed better than nanopaper I with a high proton conductivity of 14.2 mS cm⁻¹ and power density of 19.1 mW cm⁻² at high temperature (80 °C) operation in PEFCs, along with excellent durability even for 24 h of operation.

Carboxycellulose nanofibers (CNFs) promise to be a sustainable and inexpensive alternative material for polymer electrolyte membranes compared to the expensive commercial Nafion membrane. However, its practical applications have been limited by its relatively low performance and reduced mechanical properties under typical operating conditions. In this study, carboxycellulose nanofibers were derived from wood pulp by TEMPO oxidation of the hydroxyl group present on the C6 position of the cellulose chain. Then, citric acid cross-linked CNF membranes were prepared by a solvent casting method to enhance performance. Results from FT-IR spectroscopy, ¹³C NMR spectroscopy, and XRD reveal a chemical cross-link between the citric acid and CNF, and the optimal fuel cell performance was obtained by cross-linking 70 mL of 0.20 wt % CNF suspension with 300 µL of 1.0 M citric acid solution. The membrane electrode assemblies (MEAs), operated in an oxygen atmosphere, exhibited the maximum power density of 27.7 mW cm⁻² and the maximum current density of 111.8 mA cm⁻² at 80 °C and 100% relative humidity (RH) for the citric acid cross-linked CNF membrane with 0.1 mg cm⁻² Pt loading on the anode and cathode, which is approximately 30 times and 22 times better, respectively, than the uncross-linked CNF film. A minimum activation energy of 0.27 eV is achieved with the best-performing citric acid cross-linked CNF membrane, and a proton conductivity of 9.4 mS cm⁻¹ is obtained at 80 °C. The surface morphology of carboxycellulose nanofibers and corresponding membranes were characterized by FIB/SEM, SEM/EDX, TEM, and AFM techniques. The effect of citric acid on the mechanical properties of the membrane was assessed by tensile strength DMA.

This work underscores the relationship between the mechanical properties of anion exchange membranes and pH, which together can have a profound effect on the power output of the fuel cells.